In an ideal television system the light output produced by a kinescope would be linearly related to the light applied to a camera pick-up tube. In practical systems, neither the camera tube or the display tube are linear devices. In other words, the signal voltage produced by a camera tube is not linearly related to the light that is detected and the light produced by a kinescope is not linearly related to the cathode drive voltage applied to it. The relationship between light input and signal output for the camera tube, and the relationships between signal input and light output of the picture tube, are both commonly expressed by the term "gamma" which, simply stated, is the exponent or "power" to which an input function (X) is raised to produce an output function (Y). If, for example, an input function X is raised to the first power (gamma=1) to produce an output function, then the two functions are said to be linearly related. If the output varies as the square of the input function, the value of the exponent (gamma) is equal to "2". If the output varies as the square root of the input function, the "gamma" or exponent equals 0.5. Gamma, in other words, is simply a measure of curvature of a transfer function,
FIG. 1 shows the gamma of various aspects of a video signal transmission system, with curve 1a representing the transfer characteristic of the transmission side, curve 1b representing the transfer characteristic of the picture tube (kinescope or "CRT"), and curve 1c representing the overall transfer characteristic.
The transmitted video signals of the NTSC, PAL and SECAM television standards have a gamma of about 0.45 to 0.5 while the picture tube (kinescope) of color television receivers have a gamma of about 2.8 to 3.1. As a result, the overall transfer curve (light into the camera to light output from the picture tube) is not linear and the overall gamma is, in practice about 1.35 instead of a unity (1.0) gamma. The implies that the exponential transfer characteristic of the picture tube is not fully compensated, leading to compression of dark picture portions of the display. Such compression causes picture details near black to be lost, and colored areas to fade to black. Concurrently, whites are excessively amplified with respect to the dark portions to the point of often reaching picture tube saturation and blooming.
A linear overall transfer characteristic avoids the problem of black compression and can be obtained by an additional gamma correction of about 0.8 in each of the red, green, and blue (R,G and B) signal processing circuits in the television receiver. However, picture tubes have a relatively small dynamic range of light output which can not be enlarged without reaching picture tube saturation causing blooming. Therefore, gamma correction to increase amplification of dark image areas can cause a signal compression of the high signal whites. This is illustrated in FIG. 2A showing a gamma corrected ramp signal. Peak white must be kept at the same level as in the uncorrected case, the dashed line, to avoid picture tube blooming. As a consequence, the upper portion of the ramp signal has a reduced slope as shown in FIG. 2B. This corrects the black compression problem while avoiding the problem of "blooming" (excessive whites).
Reducing the upper portion of the ramp signal to avoid blooming, however, can create another problem. The viewer perceives the reduced signal as a lack of contrast in grey to white picture areas resulting in "washed out" appearing pictures. In such an event, the improvement of contrast of low-brightness portions of the image by gamma correction is obtained at the expense of high brightness contrast deterioration.
A very effective solution to the problem of providing gamma correction while avoiding loss of high brightness contrast is described by Haferl et al. in U.S. Pat. No. 5,083,198 entitled NONLINEAR RGB VIDEO SIGNAL PROCESSING which issued Jan. 21, 1992.
FIG. 3 herein is an exemplary embodiment of a television receiver (indicated as 300, generally) including kinescope driver circuits 308, 310 and 312 in accordance with an embodiment of the Haferl et al. system. The receiver 300 includes an antenna input terminal 302 that supplies RF input signals to a tuner, IF amplifier and detector unit 304 which produces a baseband video signal S1. A chrominance/luminance signal processor 306, of conventional design, provides functions such as hue and tint control, brightness and contrast control, matrixing, etc., and provides red blue and green (RGB) video color component output signals for display by a kinescope 314. The R,G and B signals are applied to respective cathodes 320, 322 and 324 of the kinescope 314 by means of respective kinescope driver and contrast enhancement circuits 308, 310 and 312. The details of circuit 308 are shown in the drawing. Circuits 310 and 312 are identical to circuit 308 and so are shown in block form to simplify the drawing.
Driver apparatus 308 of the Haferl et al. system includes an inverting, high voltage, kinescope cathode driver amplifier 330 having an input 332 coupled via an input resistor R1 to input terminal 334 (to which the Red video signal is applied) and having an output 336 coupled to the red cathode 320 of kinescope 314 and coupled also back to the amplifier input 332 via a feedback resistor R2. These elements, R1, R2 and inverting amplifier 330, connected as described, provide linear amplification of the video input signal V1 at input 334 with a gain equal to the ratio of the feedback resistor R2 divided by the value of the input resistor R1.
The remaining elements of drive circuit 308 provide non-linear processing of the input signal V1. Specifically, signal V1 is applied to a non-linear signal splitter 340 which splits the input signal V1 into a low level portion V2 representative of black to grey regions of the image and into a high level portion V3 representative of gray to white portions of the image. The low level or dark portion V2 is applied via resistor 342 to the summing input 332 of amplifier 330 and so boosts the picture brightness in the black to gray region. This provides gamma correction of dark scenes and so improves the low light contrast of displayed images. The higher level signal V3 is AC coupled via resistor 346 and capacitor 344 to amplifier 330 for improving large area contrast and is also AC coupled via capacitor 348, resistor 350 and high pass filter 352 to amplifier 330 for improving small area white contrast. Blooming is prevented by the AC coupling and high pass filtering of the gray to white picture signal applied to the high voltage cathode driver amplifier 330. Advantageously, this "dual level" processing enhances detail for both bright and dim areas of displayed images, gamma is more closely corrected and spot blooming is avoided.